When drilling through a rock formation or other formations for oil, natural gas or other materials, it is beneficial to determine or estimate the type of formation that is being drilled through. For example, information regarding the formation type can be useful for more accurately interpreting other well logging measurements in order to estimate formation porosity, water saturation, net hydrocarbon content, and formation permeability and production rates. It can also be useful in making drilling decisions based on estimated mechanical properties of the formation.
There are various ways to try to determine or estimate the formation, one of which is logging while drilling or LWD. LWD is the measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly.
One known technique of LWD is neutron-gamma spectroscopy, which uses neutrons to create excited states in a nucleus which then may decay via emission of one or more gamma rays. The gamma rays of each isotope have a characteristic energy spectrum which can be used to distinguish the concentration of a particular element such as hydrogen. As a result, the gamma rays of a group of isotopes provide a group of characteristic energy spectra that can be used to distinguish the concentrations of corresponding elements, such as hydrogen, chlorine, iron, silicon, calcium, sulfur, titanium, aluminum, sodium, magnesium, manganese, and nickel.
One drawback of the use of neutron-gamma spectroscopy is that one or more of elements in the spectroscopy tool can be the same as one or more elements in the formation such that the tool, when attempting to determine or estimate the elements in the formation, can actually also detect gamma rays resulting from elements in other than the formation such as elements in the tool or the drilling mud. That is, the spectrum for an element that is present in the formation and in, for example, the tool is actually the total of the spectrum of the element from the formation and the spectrum of the element from the tool. The portion of the spectrum from other than the formation is often referred to as the background spectrum.
Another drawback is that the environmental conditions of the formation can vary from well to well and also as a function of depth within a particular well, which can affect the spectra measured using LWD neutron-gamma spectroscopy. Examples of varying formation environmental parameters are borehole size, rock formation density, borehole fluid density, hydrogen index of the rock formation, neutron slowing-down length of the rock formation, thermal neutron capture cross section Sigma of the rock formation, or thermal neutron capture cross section Sigma of a borehole fluid.
Eliminating, reducing the effects of, or compensating for either of these drawbacks would be beneficial when attempting to determine or estimate the type of formation that is being drilled through.